This invention relates to channel connecting apparatus for connecting telephone channels or lines, and more particularly channel connecting apparatus for connecting a plurality of channels for simultaneously exchanging information among a plurality of parties. Television conference system has been developed in which a conference can be made between remote conference rooms by using television image signals and voice signals just in the same manner as in a single conference room.
In each conference room of the typical television conference system, there are various terminal devices, such as a telephone set, a picture transmission system, an electronic blackboard, etc. At least three telephone channels are required for making a connection between two conference rooms having a conference with each other, that is, at least three telephone numbers must be dialed when the telephone channels are connected.
General data communication terminal devices have no function for connecting themselves with a public telephone line, a subscriber telephone line, or an internal line of PBX. These devices do not behave in the same manner as the telephone set. Some have interface functions, such as, for example, starting a telephone exchanger. A transmission of a restore signal and a selection signal, and a detection of a calling signal, is required to perform the connection between the terminal device and the telephone channel. Such an interface is made to a worldwide standard and is named a telephone network control unit (NCU) in Japan or a data access arrangement (DAA) in United States.
There are automatic response type, manual transmission/automatic response type, and manual transmission/manual response type NCU's with respect to operation types. Prior art television conference system described above comprises a combination of the network control unit capable of manual transmission and automatic reception, and a connecting device capable of connecting lines by an one touch operation by storing a telephone number of a called party, thereby enabling channel connection in channel units.
However, in such system it is necessary to provide the network control unit and the connecting device for each one of a plurality of channels, and to effect connection/disconnection or open/close operations for each channel. Accordingly, not only the channel connection operation is troublesome for skilled persons but also unskilled persons can not readily use the system. Further, at the time of automatic transmission in the prior art telephone network control unit, whether a channel is connected to a called party or not is judged by detecting whether a polarity inversion of a channel signal has been carried out or not. However, in a recent PBX (Private Branch Exchange), a polarity inverted signal is not sent back, thereby disabling the automatic transmission to a called party. There are also such defects that, at the time of completing a conference, the user merely turns off a source of power but fails to disconnect the channel or the channel is continuously reserved when a called party disconnects the channel but the calling party does not disconnect the channel.